This invention relates to threaded fastener and washer assemblies, and is more particularly concerned with a method of permanently associating a compressible washer with a threaded fastener during the formation of threads on the fastener blank.
Screw or bolt and washer assemblies have heretofore been provided by placing the washer on the screw blank prior to the formation of the threads on the blank so that the crest diameter of the threads subsequently formed is greater than the internal diameter of the washer, thus trapping the washer adjacent the head of the screw. However, with this method it is not possible to provide thread convolutions directly adjacent the bearing surface of the head. Therefore, when a compressible washer, for example a conical washer, is associated with the screw there will be no thread formation above the plane of the outer diameter of the uncompressed washer. Such a configuration will not permit the washer to be completely compressed when the assembly is inserted in a tapped aperture since the unthreaded shank portion beneath the surface of the washer will resist complete axial insertion of the screw.
There have been various attempts to solve the problem but unfortunately all such prior art attempts have required additional and costly manufacturing steps. For example, it has previously been proposed to roll the threads on a screw blank and thereafter telescopically associate the washer with the blank. In such proposals it is required that the assembly be subjected to a subsequent staking or deforming operation to insure that the washer is retained on the shank. It has also been proposed to roll the screw blank with a shallow thread formation including a crest diameter not greater than the inner diameter of the washer, and thereafter telescopically associate the washer and screw. This operation requires the screw-washer combination to be subjected to a second and final thread rolling operation to produce fully formed threads on the shank below the washer.